It is known, in order to obtain optimum reinforcing properties conferred by a filler in a tire tread and thus a high wear resistance, and it is generally advisable for this filler to be present in the elastomer matrix in a final form which is both as finely divided as possible and as homogeneously distributed as possible. In point of fact, such conditions can only be achieved insofar as this filler exhibits a very good ability, on the one hand, to be incorporated in the matrix during the mixing of the elastomer and to deagglomerate and, on the other hand, to disperse homogeneously in this matrix.
In a known way, carbon black exhibits such abilities, which is generally not the case with inorganic fillers, in particular silicas. This is because, for reasons of reciprocal affinities, these inorganic filler particles have an unfortunate tendency, in the elastomer matrix, to agglomerate with one another. These interactions have the harmful consequence of limiting the dispersion of the filler and thus the reinforcing properties to a level substantially below that which it would be theoretically possible to achieve if all the bonds (inorganic filler/elastomer) capable of being created during the mixing operation were actually obtained; these interactions tend, moreover, to increase the consistency in the raw state of rubber compositions and thus to render their processability more difficult than in the presence of carbon black.
Ever since savings in fuel and the need to protect the environment have become a priority, it has proved necessary to produce tires having a reduced rolling resistance without having a disadvantageous effect on their wear resistance.
This has been made possible in particular by virtue of the use, in the treads of these tires, of novel rubber compositions reinforced with inorganic fillers, especially with specific silicas of the highly dispersible type, capable of competing, from a reinforcing viewpoint, with a conventional tire-grade carbon black, while giving these compositions a lower hysteresis, synonymous with a lower rolling resistance for the tires comprising them, and an improved grip on wet, snowy or icy ground.
Treads filled with such highly dispersible silicas (denoted “HD” or “HDS” for “highly dispersible” or “highly dispersible silica”), which are used in tires having a low rolling resistance sometimes described as “Green Tires” for energy saving offered to the user (“Green Tire concept”), have been extensively described. Reference will in particular be made to Patent Applications EP 501 227, EP 692 492, EP 692 493, EP 735 088, EP 767 206, EP 786 493, EP 881 252, WO99/02590, WO99/02601, WO99/02602, WO99/06480, WO00/05300 and WO00/05301.
These documents disclose use of silicas of the HD type exhibiting a BET specific surface of between 100 and 250 m2/g. In practice, an HD silica having a high specific surface acting as reference in the field of “Green Tires” is in particular the silica “Zeosil 1165 MP” (BET surface equal to approximately 160 m2/g), sold by Rhodia. The use of this “Zeosil 1165 MP” silica makes it possible to obtain good compromises as regards tire performance, in particular a satisfactory wear resistance and a satisfactory rolling resistance.
The advantage of using a silica having a high specific surface lies mainly in the possibility of increasing the number of bonds of the silica with the elastomer and thus of increasing the level of reinforcement of the latter. This is why it appears advantageous to use, in rubber compositions for tire treads, silicas having a high specific surface, possibly greater than that conventionally used of the order of 160 m2/g, in order in particular to improve the wear resistance of these treads. Nevertheless, the dispersibility of the filler and the increase in its specific surface are regarded as contradictory characteristics. This is because a high specific surface supposes an increase in the interactions between filler items and thus a poor dispersion of the filler in the elastomer matrix and also a difficult processability.